Objects, when scanned with a video camera (Vidicon for example) provide a wide range of signal frequencies which vary depending on the features and detail of the object. Any sharp in-focus features of an object, such as edges, corners and detailed texture produce high frequency signals whereas an object or portions of an object which are featureless, such as smooth planer surfaces with no edges or corners, give rise to signals of lower frequencies or even direct current signals.
A known method to help determine when an object being recorded by a video camera is in focus uses a high pass electronic filter to isolate the higher frequencies. When the lens of the camera is focused, the level of the total maximum high frequency energy of the object also varies. From a camera's view, the sharper (more in-focus) the detailed features of an object, the higher the level of high frequency energy. This level is measured and when it peaks, the detail of the object will be its sharpest and the object will therefore be as well focused as possible with respect to the video camera. This technique of using high frequencies to determine the focus status of an object is described in U.S. Pat. No. 4,584,704 issued Apr. 22, 1986.
The U.S. Pat. No. 4,584,704 discloses a spatial imaging system which uses a video camera and other electronics to distinguish objects of various shape lying within the camera's field of vision. A focal plane of shallow depth is periodically swept from a near to far range by a focus sweep servo so that different thin planes of the field of vision will be in focus at successive time intervals during the sweep. The video camera scans (in a conventional manner) the focal plane at each position, thereby producing a plurality of two dimensional images at successively different ranges. Each of these images will only sharply present the object in focus if the object (or part of it) lies in the plane at which the lens is focused. The location of any object detail (as determined by the scan lines) of these 2-D images on these planes will yield high frequency peaks. Since the position of each two dimensional image (or plane) is known, an image map of the three dimensional object can be reproduced and the object accurately identified. However, if there is no (or little) surface or edge detail on all or portions of the object then the video images of such portions will provide little or no high frequency information even when they are in focus. In such cases, the shape of the 3-D object may not be accurately reproduced and, therefore, inaccurately identified.
There are other methods for detecting shapes of 3-D objects involving the use of projected structured light patterns. By projecting a grid pattern at a ball, for example, and studying the deformations of the lines of the grid, the spherical shape may be approximated. These systems are not accurate for distinguishing similarly shaped objects and require an observer. It is therefore an object of the present invention to provide an improved method of focus detection for use with a spatial imaging system as disclosed in U.S. Pat. No. 4,584,704, whereby a projected pattern always remains in focus with the focal plane of the video camera to enhance the high frequency component, thereby providing object identification more easily and more accurately.
A general object of the present invention is to provide a method for focus detection using a video camera or other detector.
A more specific object of the present invention is to provide an improved method for focus detection using a video camera (or other detector) by projecting a defined pattern onto the surface of an object to enhance a selected frequency component, thereby permitting the selected frequency peak detection more easily and more accurately.